Gas chromatography (GC) relies upon the chemical equilibria of analytes between a mobile phase and a stationary phase in a GC column to bring about a temporal separation of the analytes in a gas mixture into a series of elution bands. Most current methods of gas chromatography rely on an open capillary tube with a stationary phase coating the inner wall of the tube to generate chemical separations. In addition, GC columns that are packed with a support that can be coated with a stationary phase can achieve chemical separations. However, eddy diffusion and channeling, due to the presence of particles, can lead to band broadening and poorer detection limits in packed columns. Therefore, open capillary columns generally offer improved separations with higher resolution, reduced time of analysis, and improved column efficiency as compared to packed columns. Furthermore, because of their open geometry and lower flow resistance, a capillary tube has a lower pressure drop, enabling longer columns to be used. For conventional bench top methods of gas chromatography, these columns are very long, usually 30 to 60 meters for open capillary tubes, as compared to as short as 2 meters for packed columns. However, narrow-bore capillary columns have a low sample capacity and working range, due to the low volume of stationary phase present in the column. In addition, injection and detection must be fast enough to take advantage of the reduced band broadening obtainable with a fast capillary column. A compromise solution to obtain fast separations with acceptable sample capacity is the multi-capillary column, comprising a bundle of narrow-bore capillary tubes running in parallel. Fast separations are achievable because narrow-bore capillaries are used, while the dynamic range can be high. However, each capillary of the multi-capillary column must have the same diameter, length, and thickness of stationary phase coating to prevent band broadening and loss of efficiency. In practice, the optimum column type and column operating parameters depend on a number of factors, including the complexity of the sample mixture, resolution required, analysis time and sample capacity desired, and pressure drop acceptable.
Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed based on gas chromatography to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. In particular, on-site monitoring with a portable system results in much shorter analysis turn-around times and can reduce the risk of contamination, sample loss, and sample decomposition during transport. These microanalytical systems should provide a high chemical selectivity, to discriminate against potential background interferents, and the ability to perform the chemical analysis on a short time scale. In addition, these systems should be small, lightweight, and require low maintenance and low electrical power consumption as are needed for prolonged field use. However, to achieve these objectives, resolution and sensitivity are often compromised. See, e.g., Frye-Mason et al., “Hand-Held Miniature Chemical Analysis System (μChemLab) for Detection of Trace Concentrations of Gas Phase Analytes,” Micro Total Analysis Systems 2000, 229 (2000).
Both open and packed in-chip channels have been used with current GC-based microanalytical systems. In particular, etched silicon channels are commonly used for microfabricated GC columns. Anisotropic wet etching or reactive ion etching can be used to form high-aspect-ratio rectangular channels with precisely controlled channel depth and width in a substrate. Typically, rectangular channels are about 10 to 80 microns wide and about 200 to 400 microns deep etched in the surface of a silicon wafer. For dense packing, the channels typically have a spiral or serpentine pattern in a die that is approximately one square centimeter in area. After etching, a glass coverplate is bonded to the etched silicon surface. The inside surfaces of the channel can be coated with a stationary phase material to enhance the separation of the chemical analytes of interest in the gas sample. For example, the stationary phase material can be a polymer having a specific chemical group with the proper physico-chemical interaction to cause separation of the analytes. Instead of using a stationary phase material to coat the surfaces of the channel, the channel can alternatively be filled with a porous packing material. Finally, the microfabricated column can be heated by a thin-film resistance heater deposited on the unetched side of the substrate. Overall column length is typically about 1 meter for open channels and as short as 10 centimeters for packed channels. See C. M. Matzke et al., “Microfabricated Silicon Gas Chromatographic MicroChannels: Fabrication and Performance,” Proceedings of SPIE, Micromachining and Microfabrication Process Technology IV, 3511, 262 (1998); G. Lambertus et al., “Design, Fabrication, and Evaluation of Microfabricated Columns for Gas Chromatography,” Anal. Chem. 76, 2629 (2004); U.S. Pat. No. 6,068,684 to Overton; and U.S. Pat. No. 6,663,697 to Kottenstette et al., which are incorporated herein by reference.
Such high-aspect-ratio rectangular channels can provide relatively high column efficiency combined with relatively high volumetric flow rates and high stationary phase surface area. This is because, with a rectangular column, resolution is primarily controlled by the channel width and volumetric flow is determined by the channel cross section. However, because of diffusion along the height dimension of the rectangular channel, high-aspect-ratio rectangular columns can suffer from band broadening. Furthermore, the rectangular geometry is difficult to coat with a satisfactorily uniform stationary phase and is sensitive to defects in the channel height that reduce the overall separation efficiency. This coating difficulty results in buildup of the stationary phase in the corners of the rectangular channel. Finally, long column lengths are required for the separation of many analytes. Simply making longer spiral or serpentine planar columns leads to large die sizes, which are not easily integrated with a microfabricated inlet system or detector of a microanalytical system.
Therefore, a need remains for a microfabricated GC column that minimizes band broadening, enables long column lengths with low pressure drop, enables uniform stationary phase coatings, and provides a column configuration that can be easily integrated with other microfabricated components to provide a compact and fast microanalytical system.